What Does Runout Mean for a Metal Lathe?

People encounter metal constantly in their daily lives – it protects you in cars, holds up buildings, provides tracks for trains, holds up traffic lights and makes refrigerators, ovens and countless other appliances. Making a metal world requires the right tools. The lathe, an important metal working tool, helps carve and finish pieces of metal for improved quality. You might encounter a handful of problems with metal lathes, including runout, which affects lathe accuracy.

  1. Metal Lathes

    • Metal lathes help you carve metal. These tools contain three basic parts: the bed, the head and the tool. The bed constitutes the flat surface that holds the tool and the head. The head holds metal in place for you while you work, while the tool holds different sizes of chucks and bits, allowing you to carve and style metal. The head allows for the rotation of metal so you can move it in a full circle as you carve it. The tool portion of a lathe slides back and forth on the surface of the bed as needed.


    • Runout means the radial variation from a true circle. In a true circle, every point on the diameter of the circle sits equidistant from the center – this distance constitutes the radius of the circle. Machine parts such as the head on a lathe very rarely rotate in a perfect circle because the smallest imperfection, such as an imperceptible bump in metal, can throw off a rotation by a fraction of a millimeter. Basically, the runout on a metal lathe constitutes the difference between the rotating motion of its head and a perfect circle.

    Problems With Runout

    • Technically speaking, probably every metal lathe in the world exhibits some degree of runout, because a perfect circle is nearly impossible to replicate in real applications. Slight runout causes few difficulties, but pronounced runout can cause serious problems. Runout can lead to innocuous issues such as cosmetic blemishes, but it also can lead to dangerous problems. For instance, a lathe with a high runout used to carve parts for a car could result in problems with wheels, axles and engine parts.

    Measuring Runout

    • You can measure the runout on lathes using specialized tools such as a dial indicator. Tim Gilles, author of the book “Automotive Chassis,” recommends never using a lathe with a runout of .003 inches in automotive applications. The book “Automotive Brake Systems” asserts that a runout of .002 used on brake systems can cause enough error to damage a car and lead to safety hazards. Contact an expert machinist or metalworker to measure runout on your lathe if you don’t feel confident doing so yourself.

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